This invention relates to the field of instrumentation. More particularly, this invention relates to non contact methods of determining the thickness of substrates, such as those used in the microelectronics industry.
As integrated circuits are fabricated to increasingly smaller dimensions, they have tended to become more sensitive to process and material variability. For example, variations in material properties such as substrate thickness and stress can effect the physical and electrical properties of the thin layers of material that are formed on the substrate surface as a part of the integrated circuit fabrication process. For reasons such as this, strict standards have been promulgated for substrate characteristics. For example, SEMI standards dictate that prime grade silicon substrates are to have a thickness of between about seven hundred-seventy five microns and about eight hundred microns. Thus, other substrate characterizations, such as stress measurements, are calculated with the assumption that the substrate thickness is within this range.
However, some processes are more tolerant of variation in substrate thickness than are others, and so in an effort to reduce the cost of the substrates used for such, manufacturers of such devices have started to purchase reclaimed substrates, which have a thickness in the general range of from about six hundred microns to about eight hundred microns. In other words, these reclaimed substrates have typically been thinned to some degree as compared to a prime grade substrate. For those that use these reclaimed substrates, and also for other manufacturers who must have a very precise substrate thickness measurement, it is not acceptable to merely assume that a given substrate falls within a given thickness range.
Thus, many manufacturers desire to perform their own thickness measurements on a sampling, or all, of the substrates that they process. Unfortunately, the traditional method of substrate thickness measurement is somewhat cumbersome. Typically, the substrate must be weighed on a mass balance with a high degree of precision. Then the thickness of the substrate is calculated using two other pieces of information, where the first is one of either an assumed or a measured substrate diameter, and the second is one of either an assumed or a measured substrate density. Of course, if the assumed values are inaccurate, then the calculated substrate thickness determination will also be inaccurate. On the other hand, if diameter and density measurements are also performed, then the already lengthy and cumbersome process of finding the thickness of the substrate is made additionally difficult, time consuming, and costly.
What is needed, therefore, is a system by which substrate thickness can be determined more accurately, more quickly, and preferably using instrumentation that a manufacturer might already own.
The above and other needs are met by an apparatus for measuring a thickness of a substrate having an upper surface, without contacting the upper surface of the substrate. A platen having a base surface receives the substrate, and a reference surface is disposed at a known first height from the platen surface. A non contact sensor senses the known first height of the reference surface without making physical contact with the reference surface. The non contact sensor further senses a relative difference between the known first height of the reference surface and a second height of the upper surface of the substrate without making physical contact with the upper surface of the substrate. A controller controls the sensor and determines the thickness of the substrate based at least in part on the known first height of the reference surface and the relative difference between the known first height of the reference surface and the second height of the upper surface of the substrate.
In this manner the substrate thickness is directly measured without contacting the surface of the substrate. Thus, there is no reliance on the potentially inaccurate assumption of a substrate density or diameter, nor is there the need to measure a density or diameter. Further, by sensing the thickness in a non contact manner, the measurement can be taken very quickly and without potential damage to the substrate. Therefore, the thickness measurement can be taken on a large sampling of substrates, or on all of the substrates, as desired.
In various preferred embodiments the reference surface is mounted to the platen.
Preferably the reference surface comprises a plurality of references surfaces, each having a known first height, where the reference surfaces are mounted at a plurality of locations on the platen. The reference surface in one embodiment has a plurality of surfaces disposed at a plurality of known heights. Most preferably the first height of the reference surface is between about six hundred microns and about eight hundred microns.
The non contact sensor is preferably an optical sensor that senses the height of a surface by focusing on the surface. Preferably there is also a coarse adjustment and a fine adjustment under the control of the controller, and the known first height of the reference to surface is sensed with the coarse adjustment, and the relative difference between the known first height and the second height of the upper surface of the substrate is sensed with the fine adjustment. In one embodiment separate non contact sensors are used to sense the first height and the second height.
A motor preferably moves one of the platen and the non contact sensor relative to each other for the different height sensing operations. The motor translates or rotates one of the substrate and the sensor relative to each other. Most preferably the motor moves the platen and the substrate relative to the sensor. The controller is preferably a general purpose computer.